Ribosomal RNAs are covalently modified during biogenesis. Failure to properly modify rRNA can affect the efficiency of protein synthesis and may be involved in tumor-formation and disease pathogenesis. One of the most prevalent rRNA modification in both eukaryotes and archaea is the 2'-O-methylation of ribose 2'-hydroxyl groups. These modifications are directed by the conserved family of box C/D small nucleolar RNAs, which contain terminal box C/D and juxtaposed internal box C[unreadable]O/D[unreadable]O motifs. At present, archaeal box C'/D' small ribonucleoproteins (sRNPs) are the only RNA-guided nucleotide modification systems that can be reconstituted in vitro to obtain enzymatically active complexes. These systems are ripe for structural investigations that determine the mechanism of complex-assembly and its linkage to regulation, and assess how mutations affect methylation activity. The long-term goals of this research are to investigate the structural biology of the box C/D-guided 2'-O-methylation systems in archaeal organisms and compare them to eukaryotic counterparts. This investigation will specifically examine the structure <--> activity relationship of the Methanocaldococcus jannaschii and Aeropyrum pernix box C/D sRNPs. In these systems, the L7Ae core protein initiates sRNP assembly by binding box C/D RNAs and establishing interactions with the Nop56/58 and fibrillarin core proteins. The goals are to elucidate how protein-RNA and protein/protein interactions, binding, folding, and assembly regulate biological activity. The specific aims are: 1. To compare and contrast the structures of the individual and juxtaposed archaeal box C/D and C'/D' sRNA motifs to those in isolated and juxtaposed L7Ae-box C/D and L7Ae-C'/D' sRNP complexes; and 2. To examine the archaeal core protein L7Ae's role in initiating sRNP assembly and the putative function of zinc in these interactions. The rationale for this investigation is to add complementary biophysical and structural data to support the mounting biochemical studies on archaeal box C/D RNA modification systems. These studies will provide insight to the evolution and development of ribonucleoprotein enzyme complexes, and illuminate the similarities and difference between the archaeal and eukaryotic sRNP systems.